A galaxy in which some kind of unusual, perhaps violent, phenomenon is releasing energy. The energy is usually released from the galactic nucleus and is not related to the way in which energy is normally released by stars. Active galaxies comprise ten per cent of the population of galaxies which can be viewed by astronomers from Earth. Many different classifications of active galaxies now exist such as Seyfert galaxies , quasars , radio galaxies and blazars. A unified theory of active galaxies exists which attempts to explain all types of active galaxy as being intrinsically the same kind of object. See also active galactic nucleus.